We report the characterization of coplanar strip (CPS) transmission lines on dielectric boards at millimeter-wave frequency up to 40 GHz. The CPS line was analyzed and simulated using computer-aided design tools. Many CPS lines were built on RT/Duroid board and the measured attenuation is as low as 0.8 dB/cm at 30 GHz. CPS lines of various electrode width and width-to-gap ratios were measured to obtain key design guidelines. The CPS line has both the signal and ground electrodes on the front surface of the board. In contrast to the microstrip line, the CSP line does not include the backside of the board as a part of the transmission line. Thus, the backside can be left open as dielectric or metallized. The backside condition can affect the wave propagation if it is too close to the electrodes. We simulated and measured the attenuation coefficient, line impedance, and electrical field distributions for different backside conditions. Our results clearly indicate that the CPS line is an attractive transmission line on the board level in millimeter electromagnetic waves applications. Its applications can be extended to microwave and millimeter-wave integrated circuits. V